Project Summary The goal of this project is to develop new, effective, and well-tolerated differentiation therapy for the treatment of patients with acute myeloid leukemia (AML). Despite remarkable advances in understanding the genomic underpinnings of AML, patients still receive the same chemotherapy that they did forty years ago. The study that established the combination of cytarabine and an anthracycline as standard chemotherapy was published in 1973(!). Immunotherapy, and novel target mutations (e.g. IDH, Flt3-ITD, Dot1L) raise the possibility of better therapies for well-defined patient sub- populations. However, differentiation therapy holds the potential of being more globally applicable for leukemias with a wide variety of underlying mutations. The finding that HoxA9 is overexpressed in 70% of AML prompted us to establish a phenotypic screening system to identify small molecules that could overcome the differentiation block established by HoxA9. A high-throughput flow-cytometry differentiation screen, followed by the molecular analysis of compound- resistant cell lines, led to the identification of the enzyme dihydroorotate dehydrogenase (DHODH) as the target of specific inhibitors that could trigger myeloid differentiation. DHODH, and its role in regulating the pool of intracellular uridine, is a novel target in AML. This unexpected in vitro finding in our engineered cell line was confirmed in human cell lines and in ex vivo PDX models of AML. We have since demonstrated that DHODH inhibitors are highly active in vivo in murine syngeneic leukemia models, human xenograft models, and PDX models of AML. This application proposes to determine the molecular mechanism through which the depletion of intracellular uridine results in myeloid differentiation. We have outlined a series of studies that build on preliminary data showing the existence of a clear therapeutic window between normal hematopoietic stem cells and leukemic cells in terms of their ability to survive periods of uridine starvation. While arguably ambitious, the proposed experiments take advantage of completed preliminary studies, allowing us to query the effect of DHODH inhibition in multiple model systems. In particular, the profiling of a 300+ panel of cell lines has yielded lines that are sensitive and lines that are 1000x more resistant to DHODH inhibition. Understanding the basis of this striking phenotype has both basic science and immediate clinical implications in terms of patient selection and disease indication. Understanding the link between pyrimidine biosynthesis and myeloid differentiation will advance our understanding of normal myelopoiesis and of how differentiation is dysregulated in the setting of leukemia. The ready availability of potent inhibitors of DHODH with favorable pharmacokinetic properties raises the exciting possibility of rapid clinical translation. David Sykes is a post-doctoral fellow in Dr. David Scadden?s laboratory at the Massachusetts General Hospital (MGH) Center for Regenerative Medicine. David Scadden is a full professor with a proven track-record of mentoring fellows in the transition to independence. The Center for Regenerative Medicine is a state-of-the-art research laboratory including microscopy and flow-cytometry core facilities as well as its own animal facility. David has established close collaborations with investigators at the Broad Institute, the Dana-Farber Cancer Institute, Boston?s Children?s Hospital, and Bayer Healthcare. These collaborators bring a wealth of co-mentorship to the project as well as scientific expertise in cancer epigenetics, metabolite profiling, medicinal chemistry, and pre-clinical drug development. David?s biosketch speaks to his academic successes. He spent a year as medical chief resident, gaining critical experience as a teacher and a leader. His preliminary research has been recognized with grants from the NIH (R03) as well as the American Society of Hematology, Alex?s Lemonade Stand Foundation, and the Leukemia and Lymphoma Society. He is dedicated to a career as a physician-scientist, and balances outpatient and inpatient clinical responsibilities (20%) with his laboratory research (80%). David is a firm believer that patient encounters help to inform and to guide basic research. To this end, he has identified an entirely new clinical syndrome (the TEMPI syndrome), and written a number of clinical case reports. David looks forward to continued mentorship as he learns to carry out and to interpret experiments in epigenetic and metabolite profiling, and to work with chemists in the rational design of small molecules around targeting critical pathways in differentiation arrest. The next years will build productively on preliminary studies in preparation for transition to an independent investigator position.